util: simplify PIPE_TEXTURE_CUBE case in util_max_layer()
util: simplify PIPE_TEXTURE_CUBE case in util_max_layer()

For cube resources, the array_size value should be 6.  So handle
that case as we do for array texture resources.  But assert that
array_size==6 just to be safe.

Reviewed-by: Ilia Mirkin <bcbbb7aa705ec3a5fc5824c01a845186c3d62fca@alum.mit.edu>
Reviewed-by: Marek Olšák <8c7344a1abdb103e79ecfd488098373070c3c70e@amd.com>

benaadams/glsl-optimizer,tokyovigilante/glsl-optimizer,djreep81/glsl-optimizer,mcanthony/glsl-optimizer,mcanthony/glsl-optimizer,zz85/glsl-optimizer,bkaradzic/glsl-optimizer,mcanthony/glsl-optimizer,djreep81/glsl-optimizer,wolf96/glsl-optimizer,bkaradzic/glsl-optimizer,zeux/glsl-optimizer,bkaradzic/glsl-optimizer,djreep81/glsl-optimizer,zz85/glsl-optimizer,zz85/glsl-optimizer,zeux/glsl-optimizer,zz85/glsl-optimizer,bkaradzic/glsl-optimizer,tokyovigilante/glsl-optimizer,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,bkaradzic/glsl-optimizer,djreep81/glsl-optimizer,zeux/glsl-optimizer,dellis1972/glsl-optimizer,jbarczak/glsl-optimizer,tokyovigilante/glsl-optimizer,jbarczak/glsl-optimizer,benaadams/glsl-optimizer,djreep81/glsl-optimizer,zeux/glsl-optimizer,benaadams/glsl-optimizer,mcanthony/glsl-optimizer,wolf96/glsl-optimizer,tokyovigilante/glsl-optimizer,wolf96/glsl-optimizer,zeux/glsl-optimizer,wolf96/glsl-optimizer,dellis1972/glsl-optimizer,wolf96/glsl-optimizer,zz85/glsl-optimizer,jbarczak/glsl-optimizer,tokyovigilante/glsl-optimizer,benaadams/glsl-optimizer,metora/MesaGLSLCompiler,benaadams/glsl-optimizer,zz85/glsl-optimizer,jbarczak/glsl-optimizer,jbarczak/glsl-optimizer,benaadams/glsl-optimizer,dellis1972/glsl-optimizer,mcanthony/glsl-optimizer,dellis1972/glsl-optimizer,dellis1972/glsl-optimizer